1. Field of the Invention
The present invention relates generally to an electrical machine for generating an output voltage, and more particularly to a generator having a permanent magnet rotor, which generator is capable of producing a voltage output which is variable over a large range in a machine that is relatively compact, efficient, and economical both in construction and in operation.
There are two basic types of generators which may be categorized by the way in which they are excited, or by the way poles are provided in the rotor. The first type of generator is the electromagnetic (EM) generator, which is excited by a direct current supplied to field coils located on the rotor, thereby producing a flux in the ferromagnetic poles of the rotor. Electromagnetic generators typically use brushes and slip rings to provide field current to the field coils located on the rotor poles. Brush-type designs are generally seen as undesirable in many applications today, since they are subject to maintenance required by normal wear, as well as for a number of other reasons mandated by the particular application.
While electromagnetic generators can be made brushless by the use of special equipment carried on the rotor (generally a solid-state rectifier fed by an exciter armature) as well as additional stationary coils (to generate a stationary field caused by direct current excitation of the additional stationary coils), the complexity of such a system is undesirable. In addition, such a system is also both expensive and unduly large in size, and the use of rotor windings result in thermal and structural problems well known in the art. Another approach used to make an electromagnetic generator brushless requires the employment of a longer magnetic circuit with inferior magnetic coupling. This approach results in a large weight penalty being incurred, and is therefore undesirable except in the case of very small generators and a limited number of special applications.
The main advantage of an electromagnetic generator is that it may easily be controlled to vary the AC output voltage of the generator. This advantage is particularly significant in situations requiring accurate control of the output voltage to compensate for changes in machine speed or load, or for changes in temperature.
The second type of generator is the permanent magnet (PM) generator, which uses permanent magnets mounted on the rotor to provide the poles on the rotor. Permanent magnet generators have a compact, rugged rotor without windings, so the thermal and structural problems of rotor windings are completely avoided. A permanent magnet generator is also intrinsically brushless, a significant advantage over electromagnetic generators.
Unfortunately, it is quite difficult to achieve a widely variable output voltage from a permanent magnet generator. The excitation of a permanent magnet generator is fixed by the properties of the permanent magnet material used in the rotor. In order to control output voltage of the permanent magnet generator, it has been necessary to greatly increase the complexity and the weight of the machine.
If only a small range of variation in the output voltage of the machine is required (on the order of 15% to 20%), then an electrical scheme which saturates or shunts a portion of the ferromagnetic circuit may be employed. When a larger degree of control is necessary (as in the case where rotor speed may vary by a factor of two), the only option has been a mechanical regulation scheme which varies the magnetic coupling of the magnets with respect to the stator or with respect to one another.
Such control schemes for permanent magnet generators are parasitic. An additional element of mechanical or electrical nature has been added, which element can reduce intrinsic excitation flux but cannot increase it. The result is a machine with a greatly reduced efficiency to weight ratio, with significantly increased costs of manufacturing and operating the machine.
It may therefore be appreciated that it is desirable to have all the advantages and features of a permanent magnet generator, with the additional feature of voltage regulation such as that in an electromagnetic generator. The machine should be of the least possible size and weight, and should be capable of highly efficient operation and economical construction. Finally, it must not be parasitic in operation, but rather selectively decrease or increase intrinsic excitation flux.